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Vacuum Measurement

Jul 25, 2019

Vacuum is the range of pressure which lies between absolute zero pressure and atmospheric pressure.

Vacuum pressures in the range of a few millibars up to atmospheric pressure are easily measured by diaphragm based pressure measurement instrumentation. In this range of partial vacuum pressures it is common to measure the vacuum using either a negative gauge reference or an absolute reference pressure sensing device. The choice of a negative gauge or an absolute reference will depend on whether you are interested in measuring the pressure compared to atmospheric pressure or to a perfect vacuum.

High vacuum pressures are very close to zero absolute pressure and much harder to measure with diaphragm type instrumentation. At high vacuum pressures it is necessary to use different sensor technology which is based on ionisation or thermal conductivity measurement principles.

Vacuum pressures are generated in many applications within the research and industrial sectors which include gas/vapour removal, leak testing, pick-up tools, sterilization, semiconductor processing and altitude simulation.

Questions & Answers

0 bar absolute = -1 bar gauge

Is zero bar absolute the same as minus 1 bar gauge?

No, only when atmospheric pressure happens to correspond exactly to 1 bar absolute which rarely happens.

Does 1 bar cover the vacuum range

Is a minus 1 bar range adequate for measuring over the vacuum range?

For most applications yes, especially if measuring low suction pressures.  If measuring a very high vacuum to some degree of accuracy, it is best to measure with a positive absolute range, since a negative gauge device will give different readings due to the changes in atmospheric pressure.

Technology used to measure over vacuum range

What type of sensing technology is used for measuring in the vacuum range?

For vacuum ranges 0-100 mbar up to 0-1000 mbar absolute it is usual to use a lower cost strain gauge diaphragm type.  For ranges from 0-1 mbar up to 0-1000 mbar absolute, capacitive or inductive sensing techniques are the typical methods for measuring higher vacuums more precisely and with less drift.   For ultra high vacuum ranges below 0-1 mbar it is not possible to measure this low a pressure with electromechanical devices and it is necessary to use less direct methods such as thermal conductivity and ionisation techniques.

Vacuum units

What pressure units are used for measuring vacuum pressures?

  • Low vacuum: psi, mbar, kPa, inHg and mmHg

  • High vacuum: Torr, Pa, mmHg, mbar

Vacuum level variations in a pumped chamber

Will the vacuum level be the same at the opposite end to the suction end of a chamber where a vacuum pump is connected?

If there is flow of gas, then you should expect there to be some difference in pressure, but it will depend on flow rate, size of volume and any restrictions between pump and end of chamber. If the suction pressure is static with no flow, then the vacuum pressure will equalise it all points.

Vacuum Gauge vs Vacuum Absolute

What is the difference between reading pressure in vacuum gauge and vacuum absolute?

Vacuum gauge is measured from ambient air pressure in the negative direction. So for example at ambient air pressure the vacuum reading is 0 bar gauge and if a suction pressure of 0.25 bar is applied, the vacuum reading will be -0.25 bar gauge.

Vacuum absolute is measured from a perfect vacuum in a positive direction. At ambient air pressure the vacuum reading will be the barometric air pressure, let’s use 1.015 bar absolute as an example. If a suction pressure of 0.25 bar is applied the vacuum reading will be 0.765 bar absolute.